1. Field of the Invention
The present invention relates to an automatic control device, particularly to a device, which detects the deviations of an electric value with respect to one or several preset values from the output terminals and feeds back the deviations to the device so as to restore the measured electric value to the preset value.
2. Description of the Related Art
The conventional three-phase inverter control device normally alternately switches 6 transistors to output appropriate current according to switching signals generated by an SPWM (Sinusoidal Pulse Width Modulation) technology. However, the SPWM technology has lower energy efficiency and higher harmonic effect. Besides, the switching frequency is unable to vary in the SPWM technology once the carrier frequency has settled. Thus, the energy consumed in the switching activities of the transistors is unlikely to reduce. Therefore, the energy conversion efficiency of the conventional three-phase inverter control device is hard to promote.
A recent paper proposed an MDFQM (Multi-Dimensional Feedback Quantization Modulator) scheme to reduce the energy lost in the switching activities of the transistors of a multi-phase inverter. Refer to FIG. 1 a diagram schematically showing a conventional MDFQM system. The MDFQM system is involved with a method for controlling a multi-phase inverter and comprises a band renormalization filter 10, an optimization unit 12, a mapping degrade unit 14, a 2D quantizer 16, and a driving signal generator unit 18, and a 2D-to-3D block 20. The driving signal generator unit 18 is composed of three-arm inverters, which control the on/off of switches S1-S6 to output voltage and current.
Several indexes are used to evaluate an inverter control device, including voltage utilization ratio, harmonic distortion, and switching frequency. The voltage utilization ratio refers to the maximum amplitude of the AC voltage output by the control device. The harmonic distortion refers to the harmonic distortion of the current signal output by the control device. The switching frequency influences the efficiency of the multi-phase control device.
Neither the conventional MDFQM system nor the conventional SPWM system feeds back the current output by the inverter. The problem may be solved with a current control circuit, which is normally realized with a PI (Proportional Integral) controller or a PID (Proportional Integral Derivative) controller. However, the abovementioned controllers consume a great amount of computation resources. Further, they are likely to have phase lag or phase advance, depending on the load. Therefore, the solution does not perfectly control the current of an inverter.
Accordingly, the present invention proposes a multi-phase inverter control device and a current control method for the same to overcome the abovementioned problems.